1. Field of the Invention
The present invention relates to a portable communication apparatus system (a mobile communication terminal) employing, for example, a direct spread-code division multiple access (hereinafter, referred to as DS-CDMA system), a circuit for detecting a shifted frequency and a method for detecting a shifted frequency, which are suitable for the DS-CDMA system.
2. Description of Related Art
According to a CDMA (Code Division Multiple Access) communication system employing a spectrum spread communication technology, a transmission side spreads a signal to be transmitted in a larger band using a spread code upon transmission. On the other hand, a reception side restores the original signal by performing reverse spread using the spread code identical to the spread code of the transmission side. This communication system is characterized by being strong in multipath fading, capable of speeding up of a data rate, its good communication quality and a high frequency usability or the like. Thus, in a next generation mobile communication system, a DS-CDMA system (a direct spread-code division multiple access, a W-CDMA system) of a development system of the CDMA communication system is employed as a wireless access system in IMT-2000.
Currently, this DS-CDMA system is standardized in 3GPP (3rd Generation Partnership Project) and application of transmission diversity is determined in its specification. It is difficult for a mobile station side to have a smaller circuit, make electric current consumption lower, make a cost lower and realize miniaturization while using an effect of space diversity sufficiently. Accordingly, in the 3GPP, in order to solve this problem, instead of allowing a space diversity reception at the mobile station side, transmission diversity for transmitting a signal by using a plurality of antennas at a base station side is employed. Depending on this transmission diversity, without complicating a structure of the mobile station side, it is possible to improve a reception property of downlink.
The above-described transmission diversity has following two systems, namely, an open loop transmission diversity for transmitting a signal from a plurality of antennas in a predetermined order pattern and a closed loop transmission diversity for transmitting a signal from a plurality of antennas after weighting at the base station side on the basis of feedback information transmitted from the mobile station side.
The closed loop transmission diversity has an effect on improvement of the reception property owing to its principle upon moving at a low speed, namely, in a case that a Doppler frequency is small. On the other hand, it is known that the reception property of this closed loop transmission diversity is deteriorated because of reception error of the feedback information and delay of tracking with respect to variation of a propagation path property at the base station upon moving at a high speed, namely, in a case that the Doppler frequency is large. Therefore, it is important for improving the reception property to detect a current Doppler frequency at the mobile station side and to judge whether or not the closed loop transmission diversity should be carried out in accordance with its size. Further, the deterioration of the reception property when the Doppler frequency is large is described in detail in a non-patent document 1 or the like.
In this case, as a method of detecting the Doppler frequency, in a patent document 1, a technique to estimate a fading pitch, namely, an inverse number of the Doppler frequency by obtaining an average time when a level of the received signal at the mobile station becomes a threshold or less is disclosed. In addition, in a patent document 2, a technology to estimate the Doppler frequency of the mobile station with respect to the base station by using a tracking correction value along with a path acquisition tracking operation is disclosed.
FIG. 9 shows a block diagram that is described in the patent document 2. In addition, FIG. 10 shows a relation between acquisition at finger processing units 101-1, 101-2, . . . , 101-n, a reverse spread value with respect to a path to be tracked, a location of a received path, and a time lapse.
The finger processing units 101-1, 101-2, . . . , 101-n shown in FIG. 9 have the same structures, respectively. In this case, taking the finger processing unit 101-1 as a typical example, the structure of the finger processing units will be described below. The finger processing unit 101-1 is made up with a reverse spreading unit 111, a path tracking determination unit 112 and a normalization unit 113. The reverse spreading unit 111 may perform a reverse spread processing for a signal that is received by the mobile station to output a reverse spread result signal 102-1a and a tracking correction value 110. The tracking correction value 110 is transmitted to the path tracking determination unit 112, and the reverse spread result signal 102-1a may be transmitted to the path tracking determination unit 112 and a RAKE signal processing unit 104 in a later stage. The path tracking determination unit 112 may determine whether or not the path can be correctly tracked using the reverse spread result signal 102-1a and the tracking correction value 110. Then, if the path can be correctly tracked, the path tracking determination unit 112 may output difference information 120 with respect to a path location before a given time together with a weighting factor 102-1c. The above-mentioned difference information 120 is transmitted to the normalization unit 113, and the weighting factor 102-1c is transmitted to the RAKE signal processing unit 104. The normalization unit 113 may obtain and output path location variable speed information 102-1b by using the above-mentioned difference information 120.
On the other hand, the RAKE signal processing unit 104 may perform RAKE combining by using respective reverse spread result signals 102-1a to 102-na that are outputted from respective finger processing units 101-1 to 101-n and at the same time, the RAKE signal processing unit 104 may estimate a relative speed to the base station by using the weighting factors 102-1c to 1-2-nc and the path location variable speed information 102-1b to 102-nb. 
In FIG. 10, a vertical axis (y axis) shows a reverse spread value to a path to be acquired and tracked by the finger processing unit, a horizontal axis (x axis) shows a location of the received path, and a depth axis (z axis) shows lapse of a time t. At the above-mentioned path tracking determination unit 112, it will be determined whether or not the path can be correctly tracked starting from t=t0, the next determination will be performed after nT hours, and then, the further next determination will be performed after (n+m) T hours.
In this example shown in FIG. 10, it is depicted that, at any one finger processing unit, a path p11 is allocated at t=t0, the path 11 is changed into a path p12 at t=t0+nT, and the path p12 is changed into a path p13 at t=t0+(n+m)T. In addition, in the example shown in FIG. 10, it is depicted that the difference information indicating a path variation amount between t=t0 to t=t0+nT is Δ1 and the difference information indicating a path variation amount between t=t0+nT to t=t0+(n+m)T is Δ2.
Generally, a following equation (1) represents a relation between a frequency error Δf and a phase difference Δφ, so that, by calculating Δ1/nT and Δ2/mT, it is possible to estimate the frequency error, namely, a relative movement speed between the base station and the mobile station.Δf·t=Δφ  (1)
In this case, a broken line in FIG. 10 represents a threshold level for determining whether or not the path is correctly tracked by the path tracking determination unit 112. In a case that the reverse spread level after the above-mentioned reverse spread processing is set less than this threshold value, the path tracking determination unit 112 may judge that the path is not correctly tracked and it may consider that there is no difference information at this point.
According to the above description, in a technique described in the above-described patent document 2, by measuring a delay profile of the path of the received signal and by using the difference information of a path timing that is periodically measured with respect to the path having a level, of which reverse spread value is set at a threshold or more, the relative movement speed represented by the frequency error is estimated.
[Non-Patent Document 1]
“Result of Outdoor Experiment of Feedback Type Transmission Diversity in W-CDMA Downlink” written by Akira FUKUMOTO, Kenichi HIGUCHI, Hashie SAWA, and Fumiyuki ADACHI, Technical Research Paper of The Institute of Electronics, Information and Communication Engineers, RCS99-156 (1999 November)    [Patent Document 1]    JP-A-2001-285129 (FIG. 5)    [Patent Document 2]    JP-A-2001-298395 (FIGS. 1 and 2)
However, when actually using a detection circuit and a method for detecting the Doppler frequency according to the above described conventional example to judge whether or not the closed loop transmission diversity operation should be carried out in the mobile communication system in the DS-CDMA system, there are some problems as follows.
As described above, in the DS-CDMA system, in an effort to secure speeding up of a data rate and a high communication quality even under an environment of a low reception level, the application of the transmission diversity is decided. On the contrary, the environment of the low reception level is assumed for judgment whether or not the closed loop transmission diversity operation should be carried out.
On the other hand, the technique described in the above-described patent document 1 requires an average time that a level of the received signal is set at the threshold and less. Therefore, a fading pitch estimation unit for estimating a period of level drop of the received signal is added as a circuit, and this results in a large circuit size of a receiver. In addition, under a communication environment where the level of the received signal is low, because of influence of noise, the Doppler frequency cannot be measured with a high degree of accuracy.
In addition, according to the technique described in the above-described patent document 2, as described above, a delay profile of each path of the received signal is measured and by using the difference information of a path timing that is periodically measured, with respect to a pathhaving a reverse spread value with a level not less than the threshold, a relative moving speed is estimated. However, there may be no pathhaving a reverse spread value with a level not less than the threshold, in a communication environment with a low reception level, or in a multipath propagation environment, and in this case, it is not possible to measure the Doppler frequency with a high degree of accuracy.
Further, as obvious from the equation (1), since the difference information may also include an amount corresponding to the difference information of the path timing derived from a carrier offset frequency error between the base station and the mobile station, it is not possible to measure the Doppler frequency with a high degree of accuracy leaving the system as it is.